Continuous process for making urea



O Ct- 21 1958 K. l. GLAss ETAL CONTINUOUS PROCESS FOR MAKING UREA oct.21,1953

Filed Aug. 29. 1955 K. l. GLASS ErAL 2,857,431 CONTINUOUS PROCESS FORMAKING UREA s sheets-sheet 2 VEA/rms v Mmmm/@ucd .wln /5 24 l, f f f /Zgl IN VEN TORJ ms; 2J/5M 0ct. 2l, 1958 K. l. GLAss ErAL 2,357,431

CONTINUOUS PROCESS FOR MAKING UREA Filed Aug. 29, 1955 3 Sheets-Sheet 3United States Patent() 2,351,431 lcoN'nNUoUs PROCESS Fon MAKING UREAApplication April 29, 1955', serialV No. 504,948 6 Claims.V (C1. 26o-555) The present invention is concerned with an improved continuousprocess for making urea by the reaction of ammonia, sulfur and carbonmonoxide.

In the copending application of Applegath et al., Serial Number 437,882,now abandoned, assigned to the present assignee, there is disclosed anentirely new process for making ureas by reacting carbon monoxide,sulfur and ammonia or an amine, with the reaction being preferablycarried out in the presence of a solvent such as methanol. As one meansofcarrying out the new process disclosed in the aforementionedapplication, it is indicated that the reactants may be fed into the baseof a. packed column together with carbon monoxide. Product urea'insolution is removed from the top of the column and separated.

The solvent is recovered for re-use, and the hydrogen sulfide and excessammonia recovered in the form of ammonium sulfate and elemental sulfur.The present invention is concerned with improvements in the urea processdisclosed in the aforementioned application.

While the cocurrent reaction of ammonia, sulfur and carbon monoxidein apacked column, as disclosed in the copending application, produces goodproduct yields of urea reconomically as compared Vwith the conventionalprior art processes involving the reaction of ammonia and carbon dioxideunder high pressure and temperature, we have found that this new ureaprocess canbe still further improved from the standpoint of operationalsteps to. better adapt the process to continuous commercial opcrations.Y

. It is. accordingly aprincipal object of the' present invention toprovide an improved continuous process for making :urea in higlr yieldsand degree: of purity byV the reaction of ammonimsutfur andfcarbonmonoxide.

Afurther object, is to provide an improved continuous process whereinsolvent, unreacted ammonia and by-product hydrogen sulfide carried inthe product solution from the .reaction zone are returned tothe reactionsystem continuously, without resort to costly recovery steps.

A still further object is to provide an improved continuous process formaking. urea wherein the loss of reactant ammonia from the reaction zoneis reduced to a minimum without the necessity of introducing undesirablecontaminants to the reaction system.

A further object is to provide animproved continuous process for makingurea wherein the product urea is produced in a high state of purity andin a form adaptable for making prilled urea for fertilizerapplications.

Another important object is to provide a Ycontinuous process for makingurea by the reaction of ammonia, carbon monoxide and sulfur wherein theloss of solvent from the reaction system is reduced to a Y Other objectsand advantages of the present invention will be apparent from thedescription which follows.

. While it is known that urea can` be lprepared by react# ing carbonylsultdezwith ammonia,commercialization of this process to the extentdesired has been retarded. 1 Pos-Y sibly the main reason for this isvthat the. cost of making the carbonyl sulde is undesirably high.

Based on the discoveries of Applegath et al. as set forth in theircopending application referred to, theuse of car# bonyl sulde as astarting material is obviated, and ureas produced directly from carbonmonoxide, sulfur and ammonia or an amine. Although the mechanism of thisre# action is not understood, in accordance with the, present improvedprocess, yields of ureaconsistently better than 90%, based on ammoniafed, are possible, with yields resulting approaching 100% based onsulfur fed. Furthermore, with our improved process, we are able toproduce nearly 100% pure urea at an improved rate. l

In carrying out the process of the present invention,v

ammonia and sulfur in solution are introduced to the reaction zone of aperforated plate columniwhich Ywill be described in more detailsubsequently. The ammonia-sulfur solution is heated to urea-formingtempera-Y tureand introduced into the column at a point so as to owcountercurrent to` a streamY of carbon monoxide introduced at the lowerend of the column. The reaction zone is maintained at a temperature ofabout 100 C., and the carbon monoxide is introduced under pressurewhichV for best operating eiiciency, is atleast equal to the vaporpressure of the. solvent mixture at urea-forming temperature. Formaximumyields, however, Vthe carbou monoxide is introduced at about 250p. s. i. g. wherein methanol is employed as the solvent. Pressuressubstantially above this are not only uneconomical, but :are unnecessaryfor maximum yields with this solvent.V The carb on monoxide ispreferably introduced in excess of the stoichiometric amounts neededvfor the reaction.

The plate column which weremploy in our improved process is divided intotwo sections.V The portion of thev column from the point of entrance ofthe feed solutionV to the bottom thereof constitutes the reaction zone,with the upper portion constituting a recovery zone where the unreactedammonia and solvent vapors tending to escape from the reaction zone arereturned thereto in a manner.

more fully described in connection with the drawings.

latter being returned to the reaction system. e

Figure 1 of the drawings show. diagrammatically theV preferred manner of.carrying out the improved process of' the present invention.

-. Figure 2 is a vertical sectional view of the plate columnV shownVdagrammatically in Figure 1, illustrating the inner arrangement ofthereaction and recovery zones of theY column.

Figure -3 is an enlarged detailed partial section ofthe v reaction zoneof Figure 2, showingfthe manner in whichthe solvent solution passescountercurrent to the movement of the carbon monoxide through thereaction zone.

Figure 4 is a top view takenY along the line 4 -4 of Figure 3, showingin detail the construction of the plates in the column.

Figure 5 shows, diagrammatic'ally a partially modified;

process Vfor making urea in the form of a saturated solution fordelivery toa pnlling tower, with-only that portion4 of the processrepresenting a modification over Figure 1 being set forth.. Y

Our improved process will-now'be described in more YYdetailwithreference rst to Figures .1, 2,'3 and 4 of the A column 10,comprisinga reaction zone 12' and a recovery zone 14, is Y provided witha Vseries .of perforatedplates 16 spaced. conveniently abput 'a foot,

drawings.

mixture with the;

. .23 apart throughout .the lengthv ofthe columnA 10. For accuratecontrol `of the temperature in the column, electrical heating elements,or steam coils may be employed around the reaction zene12 of the column.Othersheating; arrangements; may{be/remi:lovedv as desired. r Infviewof;thee-factgfhowever, Fthatj the fammonia-sulfur solution is:introduced .tof the 'i reaction zoneat urea-forming;y ternperature,the*` exothermic `naturefof thefreaction is-y usually sucient. to*`maintain f the, properV temperature; =in1 this zone lwithout resort -toexternal heating-means. Thetre'- covery "zone14of fthe .column I10.- is.water-'jacketed 'for cooling purposes. f

The a'mrnonia--sulfursolventV .mixture" isp-:prepared fin feed Vmake-upf-.tank.f18. :L :Ammonia: is fedinto` 'theritank 18;'fwhich isf.:initially 'charged Y;withi av suitable; solvent, preferably:methai'ioli;` Sulfur-fis also. charged ltot-the tanlr 18;Yeitherrinanroltenzconditionforiin a nelyfdispersed pipes20 and 20' form.:A smallamountrof hydrogen sulfide is-alsofprefer ably.: introduced:'to; lthe tank'l tof; increase" the solubility of thegesulfur in; vthesolvent.. :The :'feed .solution may containrfrorn l*lf%^fto 730%..sulfur, :butpreferably from 130 Cnwithwabout 75,C. being preferredasuthemaximmh-,temperature fors-.introducing they ffeed solution. -A\

temperature aintexcessoffV 7.5

creasez'in the vapor pressure of the ammonia inY the solution.v.Although noticritical, the liquid reactants areintroducediintoithe:top'rof the reactionY zone 12 ofzthe C. causesanexcessivem-V columnf'fl) approximately midway between the top platelnofthis zone Aand the, lower. platefl" oftheA recovery zone 14. Aspreviously indicated the reactionzone is maintained at a temperaturepreferably between 90- 105"Y C.,. although slightly lower and highertemperatures arev permissible.

While the` above arrangement for introducmg y.the feed solution iispreferred: from the standpoint-of `sunplicity and :icompactnessgfthereiare certain 'f advantages to. rfin-f troducing a portion oftherequired .ammoniaftonth'e reactiorr zone.; zcseparately.' We. have:found-,zthatif aboiltttils :o f. the f ammonia required.:is..introducedt to thereactionsizone. withnthensolvent-sulfur solutioninlthe manner.:-v described;. and :the i remaining i :ammonia:4.introduced to the columnat.someupointfnearer zth'etlower endoftheflr'eaction zone,..the vaporpressurev of'thefed solution. .issubstantially reduced; permitting r theif-intro-v duction of the feedsolution at a.temperaturecloserV to the! optimumiureaforming.temperature maintained: inl the reaction zone l12. In iaddition,such;.an. alternative; ar.

rangementereduces theamountiofn ammoniatending ',to.V

escape into the recoveryzone 14.

`Carbon monoxide is fed to .the bottom'ofi-the reaction zone-2`at;alpointfbelowlthe'. last platewl. The carbon monoxide is' introduced atpressure .dependent v.to 1. a

degree.` uponl the ivapor. pressure 'lofv .the ammonia-sulfur solution.A? number-.oflsolventsare .set forth-"inrthecopenclingilapplica'tion'referred to.as s'uitablesforV carry. ing outthe reaction. When methanol as solvent,apres-.

ah-pressure? ofifat least. 175-p..s. f i. Ig. representingcthe plates 16and 16'. It will be observed that the Weir pipes 26 and 20' extendthrough plates 16 and 16' `alternately on opposite sides thereof, eachbeing provided with a series of perforations 22 through which the carbon.i monoxideand other gaseousnconstituents pass upwardlyfA through thecolumn.

The feedV solution introduced atthetop' of the'reaction' zone 124flowsdown through'the reaction: zonergby'` gravity, in the form of aseries-of spaced'liquidcolumns 24 through which"y the,carbonmonoxidepasses4 inf-its upward path through the reaction zone.The velocity of the carbon monoxide owingthrough perforations 22 Yprevents any'ow of ammonia-sulfur solution therethrough, with thelower..por tions'of the weir. pipes Z0AV in turn each being immersedinthe liquid column im-'t mediately below the plate 16 through which itextends forming a liquidr seal,` sofas:topreventpassagerof carbonmonoxide..through Vthese;Y pipes. @Eachzofcthesispaced liquid columns 24becomes-progressivelyzricher.infprod-` uct -ureav and leaner .inf-reactant:.sulfurfromfthe point v YV of entrance of'the feedsolutionfto .thezpointlofl entrance f of the carbon monoxide fed at Vthe.bottom of the'reactionA zone -12. 1 The' ammonia andisolventvapors?itendingtoescape from 'the reactiongzone .i12^.passnpwardly;throughthe. recovery zone-'14whiclLisimilartin designito' the' reaction-zone 12.l portionzoffthe.solventztogetheriwith natrace` oammonia recoveredirom-:the product'solul tionisfreturned tozthetopiofthe recoverygzoneflito forma .-series .v ospacediliquidzcolumnsrin .the same :man` i nen-as--withfthegfeedxsolution.`...'I'he...ammonial.and sol. ventfyaporss-whichpassathroughperforations 22 inlfthe recovery-zone, -are thereby absorbedby; the-:solventfand returned to the1- .-reaction-rzone12.ilExcessjlicarbon'fmon#` oxide and-by-pro'ductfhydrogenfisulidef:areiventediromf thetopof the recoverycolumn"14,' with:.the sulfurzvalue being recovered by a'well-knownmeans.,-:Ordinarily;f;the excess.A carbon. monoxide :need Ynot lbeurecovered.

Afproductzzsolution:.isaremovedafromi.ithefbottom reaction.. zone :12..comprisingxessentiallyfurea, Ifamrnonia;Y solvent,;.and azfsmall amountof byproduct hydrogenV sulfide. 1;-The@product@isolation-:is carriedthroughs-ta heated; line '.randrzashed irnthezwessel; lzltolseparatethe* ammonia; :hydrogemsulde and-.raz substantial porti'onofv solventfrom the product urea,.enough solvent bein'gleft` Y withzthe-lnrea;tonnaintanJ itziiruid'aco'nditioirforseasy V handling. ...Theimeaesolvent esohrtion: .isa carried' -to l;evapoitator-crzystallizerercohlmmls;wvhereth lu'eacrystallized;andeanradditiohal; portionzbsolvent isi-refsmoved.;:The'incrude: .urea ccrystalstsare. centrifugedzfa't 30,1

' andxdriedr-to remove-nthezrlasttraces.'of .solvent-V iruthe,

' may'ber'ecrystallized onpurifedbytreatment withozone f where aresinigradeproductis desired. i. The crildeill'ea,4

or. as puriie'd, `may also -be'employed inmaking a .prilh'ng j 'Ysolution Wherev theV urea isintended.foriertilizer.; applicaf'y Y tions.VWhererthelilrea.is-to .bei prilled;'.hovvever,1;the-V modifiedproce'ssxorliigure iisipreferred.

vaporipressureiofathesolventzmixture. at ur'eafforrningy ternperature'-isp'referre'd.T .The carbon monoxide! is preferably introduced to thecolumn at 2750 p. s. i.;.g.;l

thelfoptimumf pressure required for the-reaction. The

Vplates .treffend liareiprovided with open-ended l.Weir

The solvent,'ammonia;;:=and hydrogen'.sulideralvapors;` z from :.theiJashv :vesse'lgztigsandwA evaporator 1.28; are '1 condensed. and;Vintroduced. to. distillation columnz34 .wherei w a...substan tial:portion 1.of,.the;.solvent, .in the."naturewf` is separated.from-.the..:ammonia and/hydrogen` t.

"gather: with. aF trace; of yabsorbed ;:ammoniag isfremoyedi from; tank.36; vfoi-delivery tot `the-A recovery. Vzone Y14 5 ofv i j column 10fontthenpurpose;previously. described.:.;';1`h`efV ammonia,hydrogenisulde;andtheismall rerriaininglpor-V y tion of:..the.uunseparatedgsolvent;are;condensed. and 'tin-f troduced;`tothelcolumrr. 38:zwheretltraces.'of :cai-bon Vmonoxide,carbon..dioxide,..;.nitrogeniaetc. i; accumulated? 1- in .the solution:systemsy areavented; to the;'atmospheleV Coldisolventxfromhsurge:taille:Stiriisfintroduced 'iirtofv f the top ofcolumn.38;-togabso'rb;ny.ammonia,.hydro-. Y

respectively which .extend .through the 2 gen suliide or solvent tendingtoV escape from column 38. The vapors from feed make-up tank 18 are alsorecovered in column 38. The solvent requirements for the reaction systemare provided from surge tank 36 by way of'the recovery zone 14 and thecolumn 38 which returns the recoveredV ammonia and by-product hydrogensulde collected by the solvent to feed makeup tank 18. VNecessarymake-up solvent is added to surge tank 36; however, very little solventneed be added as one of the important advantages of our improved processresides in the low loss of solvent from thesystem, which is less than011e poundrof solvent per 100 pounds of ureaproduced under normaloperating conditions. -The amount of by-product hydrogen suliide carriedover in the product solution is suicient to solubilize the added sulfurin make-up tank 18, and it is therefore not necessary to remove thiscompound from the solution.

YWhile methanol is the preferred solvent for carrying out our improvedcontinuous process,'as indicated in the copending application, SerialNumber 437,882, now abandoned, other solvents such as amyl alcohol andbenzene may also be used. While the reaction per se will proceed in theabsence of a solvent, a solvent is required in the present improvedcontinuous process.

In column A of Table I which follows, data from a typical run ispresented to illustrate the advantages of our improved process.V ColumnB of Table I contains data from a typical run involving the cocurrentreaction of ammonia, sulfur and carbon monoxide in a ring packed columnin the manner taught in the copending application of'Applegath et al.,Serial Number 437,882, now abandoned. The improvement inpurity, yieldand reaction time together with over-all economies eiected by ourimproved process is strikingly evident from a comparison of data fromthese two runs.

Table I Grams CO/min 85 4.0 Total Grams CO 2, 300 710 Feed Solution CompPercent N Ha 25. 8 24. 6

Percent H25--." 3. 9 3. 67

Percent S 19. 2 19. 37

Percent OHaOH 61. l 52.3 Grams Feed Soln./min 18. 6 19.25 Total GramsSoln. (Feed) 8,800 2, 999 Recovery Soln. Compn Percent NH3 1. 2 NonePercent CHSOH. 98.8 None Gms. Recycle Soln./min 10. 5 None Total gms.recycle soln., grams--. 5, 040 None Product Soln. Comp'n:

Percent NHL. 3. 24 No Data.

Percent HzS. 1. 42 No Data Percent S 0.17 1. 54

Percent Urea 31.46 36.2

Percent CHQOH- 63. 7 No Data Total gms. product soln.- 12, 300 2. 528Net urea production, grams 3, 870 915 Rate of urea production, gms/min8. 1 5.87 Percent NH3 converted to urea (based on feed) 93. 4 70. 3Percent CO converted to urea (based on eed) 78. 5 68. 2 Percent Sconverted to urea (based on feed)- 99.1 84. l Percent S left in product(based on feed) 1. 6. 7 Percent S in product 0. 5,4 4. 0 Percent NH3lost in vent (based on feed) 65 N o Data Percent S utilized 1 99. 0 N oData Percent NH3 utilized 1 99. 3 No Data Product purity 99. 96. 0Reaction temp., C.. 100 99 Reaction pressure, p. 250 250 Duration ofrun, mln. 480 156 l Utilized is actual percent which would be consumedon a recycle basis.,

Figure 5 of the drawings represents a slight modification to ourimproved process as set forth in Figure 1. Urea has become increasinglyutilized in recent years as a nitrogen fertilizer. For such application,urea is usually sold in the form of a pelleted or prilled product whichmay be easily applied to the soil uniformly by mechanical spreaders. Theprocess as demake-up. The urea-solvent solution fromaish tank26 isintroduced to the evaporator 40, where additional solvent is removedfrom the urea leaving only a VSufcient amount to permit its beingpumped. Stearrl is introduced to the resulting solvent-urea mixtureirfthc ratio of about one part by weight of steam to nineteen parts byweight of urea. After introducing thev steam,

the vurea-water-solvent mixture is introduced to thedistillation column42. VThe heat introduced by the steam, plus additional heat supplied ,inthe column, 42` causes the remaining solvent to iash oft as overhead to'be condensed andV returned to the recovery zone 14 of column 10. Y

The product urea is removed fromthe bottom' of column 42 in the form ofan aqueous slurry comprising about urea andl 5% water.A This solution ispassedY through filter 44 whilehottoV remove'water-insoluble impuritiesand delivered to a prilling ltower for spraying wherein the product isdesigned for fertilizer applications. The urea-water solution must bekept heated to a' temperature yabove the freezing point ofthe urea-watersolutionv prior to sprayingwhich in the case of a prilling solution asabove described would be about 121 C.

The modified process of Figure 5 is very important where urea fertilizeris the desired end product, as we havel discovered that the introductionof steam to the urea-solvent mixture in this manner with subsequent distillation eifects substantial purification of the. `urea to a degreesatisfactory for most fertilizer applications. We are able to therebyobviate costly puriiication steps, where the urea is intended for thispurpose. Y

The process of this invention presents numerous practical advantagesover the methods heretofore employed in the production of urea. The costfor commercial production of urea by our process is substantiallyreduced, in that high temperatures and pressures are not employed, andthe recovery and recycle system is simple and compact. Furthermore,under the conditions of our process, breakdown of urea is not a problemas with many of the prior art processes which handle the urea in amolten condition. The problem of corrosion, which may be material withprior art methods, is substantially eliminated in our process inthat'the urea is handled in the form of a solvent solution throughoutmost of our process, which Vin the case of methanol, obviates Ycorrosion problems to a considerable extent.

In the specification and claims urea is intended to mean the compoundCO(NH2)2.

While our process has been described in connection with specicembodiments, it is understood that the invention is not to be solimited. Modifications within therpurview of those skilled in the artare intended to be within the scope of the invention.

Having now described our invention and the Amanner of practicing same,we claim:

1. In a continuous process for Ythe production of urea wherein carbonmonoxide is continuously reacted in a reaction zone with a mixture ofammonia and sulfur contained in an inert solvent, a reaction productmixture containing urea, ammonia,'said inert solvent and a portion ofthe .hydrogen sulfide formed in the reaction continuously removed fromthe reaction zone and urea recovered from said reaction product mixture,the improvement which comprises continuously countercurrently reactingcarbon monoxide with said mixture of ammonia and sulfur contained in aninert solvent.

2. In a continuous process for the production of urea wherein carbonmonoxide is continuously reacted in a reaction zone with a mixture ofammonia and sulfur con adapted to produce urea directlytain'edinan'inertsolvent, aire'action product mixture con- V'tainingmag-ammonia',.saidinertsolvent and aportion of the hydrogenfs'uld'evformed in'the .reaction continuously removed from .the reaction zone andurea recovered from VAsaid reaction' productLmixture; the improvementYKwhich Vccqrn'prises' continuously 'passing a'mixture of ammoniaand"su1fur contained inV an linert solvent downwardly through'zthereaction zone and` continuously passing carbon monoxide upwardly throughtheireaction zone,A the carbon monoxide* thereby intimately contactingsaid mixture in a countercurre'nt ow relationship.

.""3;"In` a continuous .process for the production `of urea whereincarbon monoxide is continuously reacted in a Vreactionrone with amixture ofY ammonia and sulfur con-V tained inan inert solvent, areaction product mixture con;

:taining urea; ammonia,'said inert solvent and a portion' of thehydrogen `sul`1deformed 'in the reaction continuously removed from .thereaction zone and urea recovered from Y Vsaidjreaction vproductmixture,A the improvement which comprises jcontinuously countercurrentlyreacting carbon monoxidewith*saidmixturebf ammonia and sulfur'con-`taine'd'in an `inert solvent/while maintaining a reactiontemperatureinj therangefof from about'Y 90A YC. to Yabout 'monoxide'with'sa'i'd mixture'ofammonia and sulfur contained in 'an'inert solventwhile maintaining a reaction Y Ytemperature in the range of Vfrom about"C.to"about C.and a reactionpressure above'atmospheric pressure and'below about25() p. s. i. g. and wherein the ammonia is' employed inthe.molar ratio of Vfrom about 2,

mols toabout 4mols for each mol'of sulfur employed and carbonmonoxide isemployed in a quantity in excess of the stoichiometeric amountneeded forthe reaction.

5. The processV as'described in claim 4 wherein" the solvent ismethanol.

6. In a continuous process for the production'ofturea wherein .carbonmonoxidel is`co'ntinuously reactedin a reaction zone with a mixture ofammonia and Vsulfur con# tained in methanol, a reaction product-mixturecontaining urea, ammoniag'methanol and -a portion'of the hydrogen tsulfide formed in the reaction continuously` removed J from thereactionzone and urea recoveredlrfromtsaid reaction product mixture,Y theimprovement which comprises continuously countercurrently reactingcarbon i monoxide with said mixture of ammonia andsulfur con-lV e tainedin methanol while maintaining a reactiontempera-V .Y

ture in the range of from about90 C.- toV about `125 C.

and areaction pressure above' atmospheric. pressure and below about 250p. s. i. g., and wherein the ammonia is employed in the molar ratio of.from about 2y mol s. to y about 4 mols for each mol'of sulfuremployedafnd carbon 1 monoxider is employed ein' a quantity in excessoif thes'toichiometric amount needed for the reaction, and; wherein'the mixtureof ammonia, sulfurtand methanol'isrheatedl to a temperature intherangeof Vfrom aboutv 60. C.,to

about 130.o C. prior to introductionof the mixture into the, t

reaction zone. y

' No .references cited.

W i MA :l

1. IN A CONTINUOUS PROCESS FOR THE PRODUCTION OF UREA WHEREIN CARBONMONOXIDE IS CONTINUOUSLY REACTED IN A REACTION ZONE WITH A MIXTURE OFAMMONIA AND SULFUR CONTAINED IN AN INERT SOLVENT, A REACTION PRODUCTMIXTURE CONTAINING UREA, AMMONIA, SAID INERT SOLVENT AND A PORTION OFTHE HYDROGEN SULFIDE FORMED IN THE REACTION CONTINUOUSLY REMOVED FROMTHE REACTION ZONE AND UREA RECOVERED FROM SAID REACTION PRODUCT MIXTURE,THE IMPROVEMENT WHICH COMPRISES CONTINUOUSLY COUNTERCURRENTLY REACTINGCARBON MONOXIDE WITH SAID MIXTURE OF AMMONIA AND SULFUR CONTAINED IN ANINERT SOLVENT.